Air heater for blast furnaces



Nov. 1, .1938. A. scHAcK 2,134,804

AIR HEATER FOR BLAST FURNACES Filed Jan. 25, 1957 2 Sheets-Sheet 1 Nev. 1, 1938. A. scHAcK 7 AIR HEATER FOR BLAST FURNACES Filed Jan. 25, 1937 2 Sheets-Sheet 2 Attorney.

Patented Nov. 1, 1938 UNITED STATES PATENT OFFICE AIR HEATER FOR BLAST FURNACES Alfred Schack, Meerbusch, near Dusseldorf, Germany, assignor to Rekuperator G. m. b. H., Dusseldorf, Germany 3 Claims.

Many proposals have been made for the construction of known types of air heaters for blast furnaces operating on the regenerative principle such as recuperative air heaters in which the conduits are provided as steel tubes. None of these proposals have however been practically successful.

Heat resisting steels have at high temperatures an exceedingly low permanent tensile strength representing only a fraction of the tensile strength determined in the laboratory at those temperatures by the usual rapid breakdown test for tensile strength, and thus heat resisting steels are not in general adapted for use at the maximum temperatures of the hot gas inlet commonly used in air heaters for blast furnaces.

The present invention has among its objects to provide steel tubes in air heaters of blast furnaces, of such form and composition as to be durable and adapted for efiective use at the high temperatures usually applied in such air heaters.

According to the invention air heaters for blast furnaces using steel tubes have the tubes so formed or constructed and disposed that in all situations in which in use of the air heater 0 the internal diameter of the heating tubes is provided of 40 mm. or under that dimension. Usually an internal diameter of less than 40 mm.

is used.

' The tube diameter used in known types of heat exchangers hitherto employed for. other purposes in which such large volumes of air are used as in the air heaters of blast furnaces has always been much greater. Having regard to the cost of production, even in the case of ordinary air heaters steel tubes of at least 50 mm. diameter and usually between 50 and 100 mm. diameter and even larger tubesare employed where large quantities of air are required.

It will be understood that the reason for em ploying a small diameter of tube according to the invention is that blast furnaces operate with air pressures of about one atmosphere superpressure and at a high temperature. If therefore a tube of large diameter be exposed interers. The applicant has found that if the tube diameter he maintained very small the internal stress set up under such a high temperature as that commonly used in the hot gas inlet of air heaters [or blast furnaces may be so low that the tubes will last for many years. In some cases however the temperature and pressure of the air required by blast furnaces are so high that the mere reduction in diameter of the tubes made merely of heat-resisting chromium steels is insufficient to meet practical requirements, for even a very small diameter of tube when the tube is used under conditions where its temperature is about but does not exceed 1000 C. and the air pressure is not more than one atmosphere would not prevent the permanent expansion of the tube when made of chromium steel.

The invention has among its objects to avoid this disadvantage by the use of heat-resisting steel having a particular high permanent tensile strength under high temperature.

According to the invention therefore the more highly st1 essed sections or parts of the tubes in a tubular air heater are made of a different steel which however has similar resistance to heat as chromium steel but in addition has a high permanent tensile strength under high temperature. Among suehheat-resisting and strong steels are austenitic chromium nickel steels or ferritic steels wh ch are not only alloyed with chromium but also with molybdenum or tungsten.

By reason however of the difficulty of shaping and the more expensive constituents of such alloys, such steels are more expensive than the alloys of similar heat resistance which have not so permanent tensile strength. Therefore next to the heat-resistant steels which have particularly strength under heat a heat-resisting steel with less tensile strength under temperature is employed according to the invention at those positions at which smaller demands are made on the material.

According to the invention moreover the drums or collectors between which the banks of the heating tubes extend are similarly constructed of small dimensions and in addition a number of small drums or collectors are provided.

According to the invention moreover the drums or collectors are not constructed as hitherto of rectangular cross-section and of flat'limiting or defining walls but they are given a form that is curved and non-angular for example as a cylinder with curved ends, a sphere or an ellipsoid.

According to the invention moreover thedrums or collectors must always have a much larger diameter than the tubes leading into them and those drums or collectors, especially those adjacent the hot gas inlet, are according to the invention made of the more expensive heat resisting steel that has high' permanent tensile strength under heat, whereby greater durability of the drums or collectors near the hot gas inlet is secured.

A tubular air heater constructed according to the invention is illustrated in Figures 1 and 2 of the accompanying drawings-which are vertical sections mutually at right angles.

In both figures a is the hottest section or length of the tubes which must be made of a heat-resisting steel having a high permanent tensile strength under high temperature such as an austenitic heat-resisting-steel and with an internal diameter of less than 40 mm. b is the next-section or length of the tubes which is constructed of heat-resisting steel not having high permanent tensile strength under high temperature, for example ferritic chromium steel, and of the same diameter as those of the first section, and the next section or sections of the tubes c is or are advantageously constructed of ordinary ingot steel. d and i are the drums or collectors into which the respective ends of the tubes a, b, may

i be welded. The walls of the drums or collectors d are curved and the drums or collectors d are formed of heat-resisting steel having a high permanent tensile strength under high temperature such as austenitic chromium nickel steel. The hot gases are admitted at e and leave at I while the cold air enters at 9' into the upper drums or collectors i and the hot air leaves the lower'drums or collectors d at h. The upper drums or collectors i are built to form a right angle and may be made of ordinary steel as the temperature oi. the heating gases at this position is so low that the permanent tensile strength of ordinary steel renders it possible to employ that material.

The lengths or sections a, b, 0 may be more than 3 and the lengths of sections of the respective tubes may be connected together by welding.

Examples for the composition of the steels (1) Austenitic chromium-nickel-steel for the collectors d (high permanent tensile (2) Austenitic chromium-nickel-steel for the tubes a (high permanent tensile strength) Per cent Ni- 22 Cr a 22 Fe 56 (3) Ferritic chromium-steel for the tubes b (low permanent tensile strength) I claim:

1. A tubular air heater for blast furnaces in which steel tubes are used whose heated ends are exposed to high temperatures, wherein for the purpose of protecting such tubes against distortion and creeping in service the air-capacity of the air heater is made up of a large number of tubes which are provided of a heat-resisting steel and with an internal diameter that is only about 40 mms. or under.

2. A tubular air heater for blast furnaces, in which the air-heating surface is formed by steel tubes, the most highly heated parts of which are exposed to temperatures of 900 C. and over, having tubes the internal diameter of which is only 40 mm. or under, the parts of the tubes that are most highly heated consisting of a mechanically heat-resisting steel that will withstand the formation of scale, such as austenitic chromium nickel steel, and the directly adjacent parts consisting of a mechanically non-heat-resisting steel on which scale will not form, such as ferritic chromium steel.

3. A tubular air heater for blast furnaces, in which the air-heating surface is formed of steel tubes the most highly heated parts of which are exposed to temperatures of 900 C. and over, the said tubes being connected at the two ends into hollow headers, the tubes, for the purpose of their -protection against distortion and creeping in service, being of an internal diameter of about 40 mm. or under and being of a heat-resisting steel and the hollow headers being constructed of a heat-resisting steel having a high permanent tensile strength at high temperature such as austenitic chromium nickel steel and in a form presenting curved and non-angular surfaces and of small dimensions, the number of headers being a multiple for the complete assembly of tubes.-

ALFRED SCI-IACK. 

